Conventional electro-hydraulic servo valve (EHSV) based fuel metering valve (FMV) systems employ a closed loop position control system. The first stage of the servo valve is typically a double or single acting flapper valve with a torque-motor actuated flapper and the second stage is a spool type fuel metering valve (FMV). These systems include a position sensor on the spool valve for position feedback and either an integrating controller or proportional controller used for control. The integrating controller assures that the steady state sensed position matches the commanded position. However, the actual position versus commanded position is still susceptible to inaccuracies of the position sensor gain and position (i.e., calibration of the position sensor to the valve position), the position sensor demodulator accuracy, channel-channel tracking and digital resolution. The proportional controller is susceptible to the above inaccuracies as well as an allowed steady state error that is a function of disturbance magnitudes and the proportional gain of the controller.
Regardless of the controller type, the accuracy of the closed loop FMV position system is very highly dependent on the position sensor accuracy. For precise metering applications such as in aircraft fuel systems, the position sensors need to be very accurate and have high resolution. While very accurate, the position sensors are typically very expensive, both in terms of time and cost. They are relatively difficult to interface with due to the mechanical interface, the hydraulic interface, the number of small gauge wires, complicated demodulation circuitry, etc. Position sensors are also prone to failure due to the reliability of small gauge wires. This failure mode leads to dual channel requirements (i.e., two separate position sensors, drivers, and motor control) and additional cost in order to meet reliability requirements.
Elimination of the FMV position feedback sensor will save money and weight. However, the lack of position feedback and the closed loop controller means that the effects of disturbances and/or the variations in forward path gain that are sensed and/or compensated in the closed loop controller will no longer be sensed and/or compensated. To negate these adverse effects, the magnitude of the disturbances should be minimized, the inherent disturbance rejection characteristics of the forward path should be maximized and the gain accuracy of the forward path should be made insensitive to the environment. In other words, the forward path must be “robust.” The forward path must also be strictly proportional since there is no feedback to prevent the divergence that would occur with an integrating forward path.
Conventional EHSV based systems use a low energy torque motor that controls a high energy hydraulic system. The motor used has high speed but very low torque and thus low energy. The low torque levels result in the motor (and thus the FMV) being substantially affected by relatively small DC torque disturbances. For example, isolation seals, relaxation of torsion spring preload, magnet MMF (magnetomotive force) variations, variations in flux path reluctance, discrete steps in nozzle pressure feedback forces, thermal induced movement of parts, etc. can affect the torque motor. The torque motor also does not support good dynamic torque disturbance rejection (e.g., current transient, vibration, etc.) and creates resonance issues. Indirect position feedback techniques such as the use of springs degrade the FMV stiction (i.e., static friction) disturbance rejection. In addition, plugged first stage hydraulics (e.g., flapper-nozzle gap contamination) will lead to total divergence of the spool position to either shutoff or max flow. Additionally, the indirect feedback approach requires an excessive position error to produce any substantial crush force if contaminant gets in the nozzle-flapper gap.
What is needed is a system that overcomes the problems of the conventional EHSV based systems as discussed above. The invention provides such a system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.